Gaseous lasers have a number of useful industrial and medical applications. In particular, the carbon dioxide (CO.sub.2) laser is exceptionally useful in medical applications. The wavelengths emitted from a CO.sub.2 laser, between 9-11 .mu.m, are highly absorbed by water and provide the laser with the ability to vaporize the water in tissue.
It is often desirable in medical or industrial applications to have a laser capable of emitting a continuous wave (CW) low power output that is also capable of emitting a pulsed high power output. For example, a CO.sub.2 laser that emits a low power CW output is capable of such medical applications as hemostasis; however, the low power CW output cannot be used to cut away hard tissue without causing significant damage to surrounding soft tissue. On the other hand, a CO.sub.2 laser that is capable of emitting a pulse having a high peak power can be used for the abalation of hard tissue. Accordingly, a laser's versatility is increased as a medical tool if it is capable of emitting a low power CW output and a high power pulsed output.
Traditionally, most gaseous lasers, including CO.sub.2 lasers, utilize a laser exciting discharge that is transverse to the laser's longitudinal axis. The discharge is often excited by a pulsed "DC" power supply. However, when using a transverse discharge geometry the cathode of the electrodes inducing the discharge is such a poor electron emitter that a positive ion current dominates in a region immediately adjacent to it, known as the cathode fall region. As a result, a positive space charge is formed in the cathode fall region. The electric fields resulting from this positive space charge cause electrons emitted from the cathode to be accelerated sufficiently so that an avalanche ionization effect occurs in the cathode fall region. Further, the outer extremity of the cathode fall region has an electron density that is so large that an electron dominated current occurs throughout the remainder of the discharge.
For the reasons given above, it is difficult to obtain a uniform discharge throughout the discharge region, and there is a tendency for an arc to form between the electrodes carrying the charge. Further, it has been found that the occurrences of a non-uniform discharge increase as lasers utilize longer pulses or higher specific energy inputs. The high electrical fields in the cathode fall region as well as the occurrence of arcing will tend to disassociate the laser gas thereby reducing the laser's life. In CO.sub.2 lasers, oxygen is freed from some of the carbon dioxide gas within the discharge region causing corrosion.
Attempts have been made to prevent non-uniform discharge and arcing by preionizing the discharge. Other attempted solutions include manipulating the voltage pulse, tailoring the shape of the electrodes, or replacing the DC power supply with a radio frequency (RF) power supply.
Forms of preionization include the use of an electron beam, flashlamp, ultraviolet (UV) radiation, or a localized radio frequency (RF) discharge. The use of an electron beam for preionization has been found to be effective; however, it is usually far too expensive to implement on a commercial level.
Attempts at preionization using UV radiation or an RF discharge have only been marginally successful. A majority of UV and RF preionization techniques provide preionization in only a small localized region within the discharge region. Thus, no uniformity through out the discharge region is possible and often some form of gas flow is required.
The tailoring of electrode and/or pulse shapes has been successful and is widely used for transverse excitation atmospheric pressure lasers (TEA lasers), as well as commercial and military lasers. Unfortunately, this method of providing a uniform discharge only works if the input pulse is kept short, typically less than several microseconds for a CO.sub.2 laser.
By using a laser with a transverse RF discharge, as discussed in U.S. Pat. No. 4,169,251, instead of a transverse DC discharge, a more uniform discharge is produced. The utilization of the transverse RF discharge minimizes the tendency for arc formation as long as the frequency of the RF field is high enough to ensure a negligible interaction of the discharge electrons with the electrodes. This occurs when the polarity of the alternating electric field between the electrodes is reversed with sufficient rapidity so that electrons in the discharge region have insufficient time to travel between the electrodes. The electrodes function only to control the electron and ion movement in the discharge using the alternating electric field and they do not provide any part of the discharge current. This eliminates the cathode fall region and the high electric fields therein. Further, this form of laser exciting discharge tends to exhibit a positive impedance characteristic that increases discharge stability and uniformity. Continuous wave (CW) operation using a transverse RF discharge is easy to achieve unless the specific energy inputs become exceedingly high. Indeed, the vast majority of gaseous lasers sold today with average powers less than 200 watts are excited using a transverse RF discharge.
Unfortunately, transverse RF discharge excitation becomes very expensive to implement if high peak powers are required. Unlike the components required for high power DC operation, which are relatively efficient and inexpensive, high power RF operation requires more exotic components, such as vacuum tube switching, etc. For example, to obtain a peak laser energy of one joule with a pulse width of 100 microseconds and an average power of only 10 watts, an RF input power of roughly 60 kilowatt peak and 60 watt average would be required. Although the cost of producing a continuous wave 60 watt laser using an RF discharge would be no more than a few hundred dollars, the cost of producing a laser power supply having the required peak power would be approximately $10,000 or more if it has the preferred solid-state design. Thus, RF transverse excitation is prohibitively expensive in many applications requiring high peak powers.
CW operation using transverse RF discharge excitation produces a stable uniform discharge. However, the components necessary for an RF excited laser are determined by the peak power requirement, and the high power components required for RF discharge excitation are extremely expensive. The prior art, as described in U.S. Pat. No. 5,097,472, in an attempt to overcome this problem, substitutes a high power transverse RF discharge with a high power DC discharge preceded by an RF preionization pulse. The technique makes possible the use of an inherently unstable high power DC discharge by providing a uniform preionization to the entire discharge region. Further, the substitution of the high power transverse RF discharge with the pulsed DC discharge and RF preionization allows a substantial reduction in the cost of components and operation.
As shown in FIG. 1, the prior art device of U.S. Pat. No. 5,097,472 has a laser vacuum envelope 110 that contains a grounded electrode 122, a shaped second electrode 124, dielectric spacers 126 and a further dielectric spacer 128. The dielectric spacers 126 set the spacing between the second electrode 124, to which a potential is applied, and the grounded electrode 122. Electrodes 122 and 124 cooperate to produce an internal capacitance C100 in the region where a transverse discharge will be established. This abovedescribed circuit also includes a second, external capacitor C102, which has a first plate connected to electrode 122. An inductor L 101 is connected between the second electrode 124 and the other plate of capacitor C102. Capacitor C102, inductor L101 and internal capacitance C100 cooperate to define an impedance matching network. A low energy RF source 134 is coupled to the network formed by capacitor C102, inductor L101, and internal capacitance C100 via a capacitor C103.
Pulse DC energy is supplied to the laser by means of discharging a capacitor C106, which is bridged by and charged from a high energy DC current source 136. A filter represented by inductance L 104 and capacitor C107, which are in parallel resonance for the frequency of the RF source 134, is connected between the DC source 136 and the RF source 134 and acts as an RF trap to prevent the transmission of RF energy to the DC source 136. In turn, the RF source 134 is protected against the DC source 136 by the capacitor C103.
The laser includes a discharge chamber 130 defined in part by a pair of spaced-apart oppositely facing mirrors (not shown). The gas discharge occurs in the discharge region between these minxors and between electrodes 122 and 124.
In operation, energy from the RF source 134 is applied between electrodes 122 and 124 in order to preionize a laser medium. Power from the RF source 134 must be applied across electrodes 122 and 124 such that a voltage in excess of that needed to establish a weak gas discharge will be present. When a discharge is established by the low energy RF source 134, a path for the discharge of a capacitor C106, or a path for a lower frequency unrectified alternating current from the DC source 136, will be created between the electrodes 122 and 124 and energy will flow from the capacitor C106 and/or the high energy DC source 136.
Although this prior art laser design solved many of the disadvantages found in transverse excited gas lasers, there are still a number of notable disadvantages that remain. Foremost is that the prior art laser cannot independently optimize the electrode shaping and spacing for both the RF and DC discharges since both must use the same electrodes 122 and 124. Also, extreme care must be taken to ensure that the RF and DC sources 134 and 136 are appropriately isolated from one another. Another disadvantage is that any attempt to minimize the cathode fall region by changing the area of the electrodes 122 and 124 will directly influence the effectiveness of the RF source 134, since the RF source uses the same electrodes. Further, the prior art laser suffers from having a discharge region that is made from disparate materials, leading to thermally induced geometric distortion of the discharge region during the laser's use. Finally, the prior art laser uses costly machined ceramic components.